Summary Cryptosporidium remains a significant AIDS-related opportunistic infection among people with late HIV diagnosis or without access to HAART. This parasite infects the gastrointestinal epithelium in humans; infection is also a common cause of diarrhea in young children in developing countries. There is currently no fully effective therapy available for the infection. Cryptosporidium has been referred as a ?minimally invasive? mucosal pathogen, and epithelial antimicrobial defense is key to mucosal innate anti-Cryptosporidium immunity. Whereas it is well appreciated that Type II interferon-gamma (IFN-?) is required for preventing development of intestinal cryptosporidiosis, recent advances have revealed a significant Type I IFN response (e.g., induction of IFN-alpha and IFN-beta) in host epithelium following C. parvum infection. The molecular mechanisms underlying Type I IFN response and its potential role in the pathogenesis of cryptosporidiosis are unclear. Cryptosporidium parvum virus 1 (CSpV1) is a member of the family Partitiviridae, genus Cryspovirus that infects C. parvum and other Cryptosporidium spp. Our recent studies demonstrate that host delivery of CSpV1-RNAs at the host-parasite interface can trigger a Type I IFN response in host cells. C. parvum infection attenuates intestinal epithelial cell response to IFN-? stimulation (i.e., infected cells are less susceptible to activation by IFN-?). Surprisingly, knockout Type I IFN signaling in intestinal epithelial cells or inhibition of CSpV1-RNA delivery can restore cellular response to IFN-? stimulation and promotes resistance to C. parvum infection, suggesting a negative impact of Type I IFN signaling on epithelial anti-Cryptosporidium defense. Based on these observations, we hypothesize that cryptosporidial infection triggers Type I IFN response in the intestinal epithelium and attenuates IFN-?-mediated epithelial antimicrobial defense through delivery of CSpV1- RNAs. We will use in vitro, ex vivo, and in vivo infection models and complementary biochemical, molecular, and morphologic approaches to elucidate the molecular mechanisms by which Cryptosporidium infection triggers Type I IFN gene transcription in the intestinal epithelium (Aim 1), determine how Cryptosporidium infection modulates the JAK/STAT signaling in infected intestinal epithelial cells (Aim 2), and define the impact of Cryptosporidium-induced JAK/STAT signaling dysregulation on epithelial anti-parasite defense (Aim 3). The proposal is conceptually innovative as it tests a novel mechanism for symbiotic CSpV1 in the pathogenesis of cryptosporidiosis and has the potential to inform future development of new therapeutic strategies.